Contrary to liquid crystal based projection displays, the Texas instruments DLP™ (digital light processing) does not require polarized light to operate. Rather the modulation of light is based on angular deflection of the light beam per pixel. And special optical architectures have been developed to couple the light from the lamp towards the DLP at the correct angle using total internal reflection (TIR) in special prism architectures. See FIG. 1 in which a lamp is shown directing light through a TIR prism to a DMD back to the prism and then to a projection lens.
Stereoscopic projection solutions have been developed based on polarization of light. Using a polarization preserving screen the left eye image is projected using one polarization state while the right eye image is projected using an orthogonal polarization state. The user wears polarizer glasses to pass only the appropriate image to the respective eye. Solutions with two projectors using passive polarization filters as well as single projector solutions using active polarization switches have been developed. See FIG. 2.
However in both cases when the light coming from the projector is unpolarized, half of the available light output is blocked. Complex optical systems have been proposed to recuperate the lost light (WO2008048494). But those add significant cost and difficulties to converge the two images with different polarizations to overlap on the projection screen.
It would be advantageous if a DLP projector could be adapted to deliver polarized light in an efficient way. This is especially valid if the light source is already polarized, which is the case for instance when laser illumination is used inside the projector. In this case, it would be sufficient to maintain the polarization throughout the optical path of the projector to enable efficient stereoscopic projection based upon polarization.
When looking at the light path of a DLP projector different elements can be identified from the light source to the projection lens 150. The light from the light source is coupled to an optical integrator 100 such as an integrator rod. At the exit of the optical integrator 100 such as an integrator rod a uniform light spot is achieved. This light spot is then imaged onto the DMD imager or imagers 140 by means of the relay lens system 110. The relay lens system consists of various individual lens elements (in this example 111 till 115) and can optionally also contain 1 or more folding mirrors 116 and an aperture 117. The TIR prism assembly 120 sends the light towards the DMD imager(s) 140. When a multi-chip architecture is used a color splitting prism 130 is placed between the TIR prism assembly 120 and the imagers 140. The color prism assembly will split the light in at least 2 spectral components, send it to the respective imager 140 and recombine the at least 2 spectral components into a single output beam towards the projection lens 150.
Even if the light at the entrance of the optical integrator is linearly polarized, this does not guarantee that the light from the projection lens will also be linearly polarized. The different components of the optical path can, and without special measures will, affect the polarization properties of the light.
When light reflects at an angle from an interface between two transparent materials, the reflectivity is different for light polarized in the plane of incidence and light polarized perpendicular to it. Light polarized in the plane of incidence is said to be p-polarized, while that polarized perpendicular to it is s-polarized. As a result a linearly polarized input beam is converted to an elliptically polarized reflected beam.
With reference to FIGS. 3a and b it is clear that such total internal reflection will occur in the TIR prism assembly 120 and that linearly polarized light can be changed to elliptically polarized light when the polarization direction does not coincide with the s- or p-plane.
Also, in the color prism assembly 130, multiple TIR reflections will occur. The color prism assembly will include 1 prismatic element per primary color onto which the DMD device 140 for that specific color is attached. For example the color prism includes a prismatic element 131 onto which the blue DMD is attached, a prismatic element 132 onto which the red DMD is attached and a prismatic element 133 onto which the green DMD is attached. The surface of the blue prism 131 adjacent to the red prism 132 is covered with a dichroic coating that will reflect the blue light and passes the red and green light. The surface of red prism 132 adjacent to the green prism is covered with a dichroic coating that will reflect the red light and passes the green light. By changing the characteristics of the dichroic coatings a certain prism element can be dedicated to process another color. For example it is possible to define the coatings such the prism element 131 becomes dedicated to the red color and prism element 132 becomes dedicated to the blue color.
With reference to FIGS. 4 to 6 the green light is transmitted through the color prism in a straight path passing through the dichroic coatings on the blue and red prisms at surfaces 131b and 132b. The reflected light follows a similar path. The reflected on-state green light 510c is incident on the dichroic coating surfaces 131b and 132b in a plane perpendicular to those surfaces. Therefore, when the reflected on-state green light 510c is incident on the dichroic coating at surface 132b with linear polarization in the s-plane or p-plane, then this condition will at the same time be met on the dichroic coating at surface 131b and the polarization will remain unaffected. The transmitted green light 500c however passes these dichroic coated surfaces under an angle (as can be seen from the side view of the prism). It then becomes impossible to define a polarization direction that lies in either the s- or p-plane for both surfaces at the same time. As a result it was found that those dichroic coatings convert the polarization state of the transmitted green beam 500c to a general elliptical polarization state.
The blue transmitted light 500a first reflects of the dichroic coating on the surface 131b and then is reflected via TIR on surface 131a, a similar path in reverse order is followed by the reflected blue light 510a. While the plane of incidence for the reflected on-state blue light 510a is perpendicular to the prism surfaces 131a and 131b, the transmitted blue light 500a hits the prism under an angle. As a result it was found that the TIR reflection has a slight polarization changing effect on the transmitted blue beam 500a. 
The red transmitted light 500b first passes through the dichroic coating on the blue prism at surface 131b. It is then separated from the green light by the dichroic coating at surface 132b and reflected by TIR at surface 132a. A similar path in reverse order is followed by the reflected red light 510b. Also here the plane of incidence onto the prism surfaces 131b, 132b and 132a for the reflected on-state red light 510b is perpendicular. While again the transmitted red beam 500b will be incident on those surfaces at an angle. And hence a combination of polarization conversion by the dichroic coating and by the TIR was found.
The above statements that the reflected on-state light 510 hits the respective prism surfaces in a plane perpendicular to those surfaces is only valid for a perfectly collimated transmitted illumination bundle 500. For practical setups with an illumination bundle of a finite f-number (typically f #2.5 for a DMD device with 12 degrees tilt angle) also the reflected on-state light will experience some polarization change.